1. Field of the Invention
The present invention relates to a laminated polyester film. More particularly, the present invention relates to a laminated polyester film which has excellent runnability and abrasion resistance and is suitable as a base film of a magnetic recording medium having good electromagnetic conversion characteristics.
2. Description of the Related Art
Recently, a recording density of a magnetic recording medium is remarkably increased and further increase of the recording density is desired. With the increase of recording density, it is necessary to reduce a thickness of a magnetic layer of the magnetic recording medium and therefore surface properties of a polyester base film have large influence on the electromagnetic conversion characteristics of the magnetic recording medium. That is, surface unevenness of the base film is reelected on a surface condition of the magnetic layer. In particular, when coarse protrusions are present on the surface of base film, they will cause drop out. Therefore, the film surface should be made as smooth as possible.
On the other hand, in order to improve runnability of the polyester film during production of the film or a winding property during winding up the film, or to keep good runnability of a magnetic tape in contact to guide pins or a magnetic head, unevenness formed on the film surface so as to impart a slip property to the film.
To achieve the high recording density and high quality of the magnetic recording medium, the film surface should be designed so that the above two contradictory properties, namely the surface smoothness and maintenance of runnability are satisfied simultaneously.
In addition to the above properties, it is highly required to reduce a production cost of the polyester film. To this end, in addition to the increase of productivity and yield, it is important to reduce a cost of a raw material. That is, a self-reclaimed or recycled material or a mass produced raw material which is produced by a continuous polymerization method has been investigated as a raw material of the laminated film.
When a cheap raw material such as a reclaimed material is used, there arise various problems that solid or gel-like foreign materials contained therein form large protrusions which can be a cause of drop out of the magnetic recording medium comprising a base film produced from such material, an intrinsic viscosity is decreased so that the productivity of the film is decreased, or the properties of the film such as abrasion resistance are deteriorated.
Today, since the recording density of the magnetic recording medium is very high, a base film which cannot solve these problems will not be used as the base film of magnetic recording medium.